Title

Author

Document Type

Dissertation

Date of Degree

2011

Degree Name

PhD (Doctor of Philosophy)

Degree In

Speech Pathology and Audiology

First Advisor

Eileen M. Finnegan

Abstract

Alteration in the tension or stiffness of the vocal folds, mediated by the cricothyroid (CT) muscle activation, is one of the laryngeal strategies attributed to devoicing during speech production. While some studies have reported a consistent increase in CT activity associated with voiceless sounds in comparison to their voiced cognates during speech (Kagaya & Hirose, 1975; Dixit & MacNeilage,1981; Lofqvist, McGarr & Honda, 1984; Löfqvist et al., 1989; Hoole et al., 2004), other studies have suggested that closer relation of CT activity to supra-segmental characteristics of the utterance rather than voicing contrasts (Hirose & Gay, 1972; Hirose and Ushijima,1978; Collier, Lisker, Hirose & Ushijima, 1979). The purpose of this study was to test the hypothesis that during intervocalic syllable production, CT muscle activity was consistently higher for voiceless sounds than their voiced cognates, to assist in vowel devoicing, when pitch, intensity, stress and aspiration were kept controlled. Simultaneous recordings of thyroarytenoid and cricothyroid muscle activity, video-nasendoscopy, and audio signals were obtained from eight, gender-matched adult speakers during intervocalic (VCV) syllable production. The speech sounds represented contrasts in manner of production including, plosives, affricates and fricatives with their voiced-voiceless and aspirated-unaspirated cognates Two voicing transition events were identified and marked on the EMG signals: Vowel-Consonant Transition (VC) and Consonant-Vowel Transition (CV).The mean amplitude of CT muscle activity during voicing transitions (VC, CV) was calculated over a 60 ms time window preceding the acoustic event. CT muscle activity was normalized to a percent of mean maximum activity and compared between voiced/voiceless and aspirated/unaspirated cognates across different categories of speech sounds, subjects and, VC and CV transitions. During the VC transition of intervocalic syllable production of voiceless and voiced speech sounds, a consistent increase in CT muscle activation was not observed across all subjects suggesting that CT activity is not essential for devoicing. Four of the eight subjects (M1, M4, F2, F3) showed consistently higher CT activity for voiceless than voiced sounds while, four of the eight subjects (M2, M3, F1, F4) either did not show any increase in CT muscle activity or showed an inconsistent pattern of increase. The magnitude of difference in CT activity between voiceless and voiced sounds ranged from 3-24% with the highest difference for voiceless fricatives, followed by voiceless aspirated stop plosives and voiceless affricates, and with least difference for unaspirated stop plosives. During consonant vowel (CV) transition of intervocalic syllable production seven of the eight subjects showed lower or comparable levels of CT EMG activity for voiceless speech sounds in comparison to their voiced cognates. Results of one-tailed unpaired t-test revealed that four of the eight participants, M1 [t (60.69) = 10.17, p < .001], M4 [t (8.88) = 57.03, p < .001], F2 [t (5.88) = 35.20, p < .001] and F3 [t (8.91) = 51.00, p < .001], showed significantly higher CT activity for the voiceless than voiced sounds during the VC transition of intervocalic syllable production but the results were not significant for subjects M2, M3, F1 and F4. No statistically significant difference was found between aspirated and unaspirated cognates for all the eight subjects on a two-tailed, unpaired t-test. No gender differences were observed in the findings.